1. Field of the Invention
This invention relates generally to the structure and fabrication process of a temperature sensor. More particularly, this invention relates to the materials employed for forming the sensor and the passivation layers, special structural features and fabrication process for producing improved temperature sensor that are more reliable for long term high temperature operation.
2. Description of the Prior Art
A temperature-sensing device manufactured by forming a thin metallic film on a substrate often presents special technical difficulties and application limitations not yet resolved by those of ordinary skill in the art. Specifically, the range of temperature measurement is limited due to the problems of device integrity when the sensors are employed for higher temperature measurement above 700xc2x0 C. (Celsius). As the sensor experiences temperature cycles ranging between 700xc2x0 C. and the room temperature, the differences in the temperature expansion coefficients (TEC) between different layers generate stress that gradually damages the layer integrity and the sensing element and bonding pads of the sensor. The mechanical and electrical connections are degraded due to the weakened layer structure. Furthermore, as the mechanical structure and the layer integrity are adversely affected through temperature cycles, the precision of temperature measurement is also degraded. Therefore, it is difficult to provide a highly reliable sensor when a thin film resistive-temperature detector (RTD) is applied to measure temperature above 700xc2x0 C. In addition to these difficulties, in order to achieve higher level of sensing precision and sensitivity, structural integrity of the sensing device is often compromised. This is often caused by the fact that as the structural integrity are improved by more securely attaching the sensing element to the supporting substrate. The materials and structural elements employed often adversely interfere with the measurements performed by the sensing element thus adding imprecision or lowering the measurement sensitivity.
In order to provide a highly sensitive temperature sensor, it is desirable that the temperature-sensing element is as thermally insulated as possible from the supporting substrate. Therefore, a substrate of very low thermal conductivity is suitable for such applications. However, a typical thermal insulating material, e.g., a glass substrate, often produces a difficulty that the sensing element, e.g., a platinum thin film cannot be securely attached to the surface of such substrate. Problems with structural integrity due to weak adhesion between the bonding pads and the substrate also cause a major difficulty in sensor reliability. Loose connection of the sensor elements from the measuring electrodes connected thereto or peeling off of the platinum thin film from the supporting substrate are often potential problems which must be taken into consideration when design a sensitive temperature sensor applying thin film technology. Particularly, poor contact can cause increase in resistance and generates errors in temperature measurement.
Many prior art attempts are applied to resolve this difficulty. jinda et al. disclose in U.S. Pat. No. 4,805,296 entitled xe2x80x9cMethod of Manufacturing Platinum Resistance Thermometerxe2x80x9d (issued on Feb. 21, 1989) a method of manufacturing a resistance thermometer by preparing a support substrate and forming a platinum film serving as a temperature measuring element. A sputtering process containing a predetermined amount of oxygen gas is applied to form the platinum. The resistance thermometer may further include an aluminum oxide film serving as a stabilizing layer to improve the stability and reproducibility of the sensor characteristics, namely the platinum layer. The aluminum layer is particularly useful for stabilizing the resistance temperature characteristics during a high temperature treatment of the resistor thermometer. FIG. 1 is included from Jinda""s patent as a background structure for understanding the technology involved in this invention. An aluminum oxide layer 2 covers a silicon substrate 1 for supporting a platinum film 3 thereon. The platinum film is then patterned and provided with lead wires 5 for bonding to the opposite ends of the patterned platinum film. Jinda et al. employs an inexpensive glass as substrate and applies the aluminum oxide layer to provide adhesion for the platinum film to attach to the substrate and heat resistance to sustain the heat treatment. Adding the aluminum oxide film however does not provide a complete solution to the difficulty that the platinum film may still peel off from the glass substrate during operation of the sensor. The platinum film does not securely adhere to the substrate because there is significant difference in temperature expansion coefficients (TEC) between these layers. The structural integrity is still a problem for a high temperature TCR type of device especially when thin film technology is applied to make miniaturized devices for high temperature measurement.
Similar device structures are disclosed in many other patents. Reichelt et al. disclosed in U.S. Pat. No. 4,050,052, entitled xe2x80x9cElectrical Temperature Measuring Resistor Structure Particularly for Resistance Thermometersxe2x80x9d (issued Sep. 20, 1977) by forming the platinum film on a layer which has a temperature coefficient of expansion matching the platinum layer. The expansion layer disclosed is an aluminum oxide layer. Frank et al. disclosed in U.S. Pat. No. 4,129,848, entitled xe2x80x9cPlatinum Film Resistor Devicexe2x80x9d (issued Dec. 12, 1978) by sputtering the platinum film on a quartz and etch the film and the quartz to define the conducting paths of the resistor device. Furubayashi et al. disclosed in U.S. Pat. No. 4,649,365, entitled xe2x80x9cPlatinum Resistor for the Measurement of Temperaturexe2x80x9d (issued Mar. 10, 1987) by forming the platinum film on a silicon substrate overlaid with an aluminum oxide film. Sittler et al. disclosed in U.S. Pat. No. 4,791,398, entitled xe2x80x9cThe Film Platinum Resistance Thermometer with High Temperature fusion Barrierxe2x80x9d (issued Dec. 13, 1988) by forming the platinum film on a silicon substrate overlaid with an barrier layer which is preferably a titanium oxide layer. All these devices, with similar structure as that of Jinda et al., are still limited by the same difficulty that the device structure is subject to integrity degradation for high temperature measurement due to TEC mismatches between layers particularly between that of the platinum film and the silicon substrate.
Therefore, a need still exists in the art of design and manufacture of thin film TCR type of improved devices for high temperature measurement with the sensor undergoing great range of temperature cycles. It is required that the novel and improved structure is able to resolve the difficulties generated by the weak structure configuration of conventional sensors. Additionally, this improved device is required for high precision temperature measurements where the sensing elements are constantly subject to great ranges of temperature cycles. The improved sensor is to provide assurance of high degree of structural integrity for long term high temperature measurements. It is also desirable that the structure integrity can be improved without sacrificing the measurement sensitivity whereby implementation of the TCR types of sensing device can be broadly incorporated in various modem high precision, high-speed applications.
It is therefore an object of the present invention to provide a new layer structure and manufacture method for manufacturing a reliable high temperature TCR thin film sensor such that aforementioned difficulties and limitations encountered in the prior art can be overcome.
Specifically, it is an object of the present invention to provide a new layer structure and manufacture method for providing a reliable and accurate thin film RTD chip sensor that can sustain long-term high temperature operations without degradation of structure integrity. The TCR film is deposited onto a pure aluminum oxide substrate. Then, the TCR film is protected by multiple-layer multiple annealing aluminum oxide layers. The residual mechanical stresses between the protective aluminum layers and the resistive sensing thin film that could be induced by temperature cycles are completely relieved by multiple annealing processes. Therefore, the TCR resistive can provide long term stable and precise temperature measurement with being unduly affected by the residual stresses induced by temperature cycles.
Another object of the present invention is to provide a new structural configuration and manufacture method for providing a reliable and accurate thin film RTD chip sensor that can sustain long-term high temperature operations. The TCR film trimming is performed after relief operation of the residual stress between the TCR thin film and the multiple-layer multiple-annealing protective layer is completed. Unlike the conventional RTD chip, in this invention, the resistive film trimming is performed with the resistive film covered under the multiple-layer multiple-annealing protective film. And for the purpose of resistive film trimming, temporary resistance measuring pads are formed only for trimming measurement.
Briefly, in a preferred embodiment, the present invention includes a temperature sensor. The temperature sensor includes an aluminum oxide substrate and a thin-film resistor having a specific temperature coefficient of resistance (TCR) disposed over the substrate. The temperature sensor further includes an aluminum oxide stress-relief layer covering the thin film resistor. The temperature sensor further includes a passivation layer covering the aluminum oxide stress-relief layer wherein the aluminum oxide stress-relief layer further having at least one resistor-trimming trench formed by removing a portion of the aluminum oxide stress-relief layer and thin-film resistor therefrom and the resistor-trimming trench is filled with a material of the passiviation layer. In a preferred embodiment, the temperature sensor further includes a set of dummy pads for resistance-trimming measurement disconnected from the thin film resistor disposed on the substrate near the thin film resistor covered by the passivation layer. In another preferred embodiment, the temperature sensor further includes a set of sensor bonding pads disposed on the substrate electrically connected to the thin film resistor covered by the passivation layer. And, the temperature sensor further includes a set of platinum chip-leads bonded to the sensor bonding pads for temperature measurement connections.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the various drawing figures.